The invention relates to a method for drying bulk material, preferably of plastic granules, in a drying hopper by means of dry air, wherein the exhaust air escaping from the drying hopper is dried in a drier containing a moisture ad-sorption means and is returned thereafter to the bulk material as dry air. The invention further relates to an apparatus for carrying out said method.
From German publication No. DE-OS 31 31 471 a method and an apparatus are known for drying exhaust air escaping from one or a plurality of drying hoppers each containing bulk material, and being fed to a drier containing a plurality of drying pots respectively filled with moisture adsorption means and for returning the effluent air from the drier as dry air through a conduit to the bulk material to be dried.
From German publication No. DE-AS 15 44 034 it is known to arrange a plurality of drying pots each containing a humidity adsorption means in such a way in an adsorption drier that their contents may be regenerated one after another at intervals by means of hot air. The regeneration cycles of the pots are offset in time in such a manner that always at least one pot having regenerated adsorption means is available for drying the exhaust air.
It is one of the characteristics of the adsorption means that the hotter it is the less it will adsorb moisture from the exhaust air. The capacity of the adsorption means to adsorb moisture is therefore only partly utilized when the drying process starts, as has been common, at a medium or high temperature. Moreover, the effluent air from the drier has a higher moisture content, i.e. it is not optimally dried.
For surmounting these drawbacks one may therefore conceive to cool the exhaust air by a water cooler before it is fed to the drier. Normally, however, cooling water is not available, i.e. tap-water has to be used causing great expenses. Furthermore, cooling is limited by the temperature of the cooling water which is frequently not colder than 25.degree. C. to 30.degree. C.